When Charles Darwin wrote On the Origin of Species, he pondered the evolution of organisms ranging from orchids to whales. Conspicuously missing from his magnum opus, however, was any substantive discussion of how humans might have arisen. He wrote only “light will be thrown on the origin of man and his history.” Scholars attribute Darwin’s relative silence on this matter to reluctance on his part to further nettle the Victorian establishment (and his pious wife), for whom the origin of all living things—especially humans—was God’s work.

Thomas Henry Huxley, the biologist otherwise known as “Darwin’s bulldog,” had no such reservations. In 1863 Huxley penned Evidence as to Man’s Place in Nature, in which he explicitly applied Darwin’s theory of evolution to humans, arguing that we had descended from apes. Eight years later Darwin himself, possibly encouraged by Huxley’s effort, wrote The Descent of Man. In it he declared the chimpanzee and gorilla our closest living relatives based on anatomical similarities and predicted that the earliest ancestors of humans would turn up in Africa, where our ape kin live today. At the time, only a handful of human fossils were known—all of them Neandertals from sites in western Europe.